Disk drives are well known in the computer art for providing secondary mass storage with random access. A disk drive essentially comprises one or more magnetic data storage disks rotating on a spindle within an enclosed housing. A magnetic transducer head is positioned very closely to each data storage surface by a slider suspended upon an air bearing. Closest clearance between the smooth disk surface and the slider is typically on the order of one microinch, or less. This close proximity of the head to the disk surface enables very high-resolution data and servo patterns to be recorded onto the disk surface. Servo patterns are typically written in servo sectors which interrupt data sectors or blocks. Servo patterns provide the disk drive with head position information to enable a head positioner mechanism, such as a rotary voice coil positioner, to move the head from track to track during random access track seeking operations, and to maintain the head in proper alignment with a track centerline during track following operations when user data is written to or read from the available data block storage areas of the disk surface.
Data transducer heads currently being used employ dual elements. An inductive write element having a relatively wide recording gap is used to write information into the tracks, whereas a so-called giant-magneto-resistive read element having a relatively narrow playback gap is used to read information from the tracks. With this arrangement, data track densities equaling and exceeding 16,000 tracks per inch are possible, leading to disk drives in relatively small packages or form factors with very large data storage capacities. One drawback of using a narrow read element relative to a wider write element is that the fine position servo information needed to position the head over a particular track becomes more complex, requiring more servo bursts (circumferentially sequential, radially staggered single frequency bursts, pairs of which are sequentially read as the read element passes by a servo sector).
Servo patterns are written into the servo sectors of each disk conventionally with the aid of a servo writer at a point in the drive assembly process before the head disk unit is sealed against particulate contamination from the ambient. A servo writer is a complex and expensive manufacturing unit, typically stabilized on a large granite base to minimize unwanted vibration and employing e.g. laser interferometry for precise position measurements. The servo writer typically requires direct mechanical access to the head arm, and may also have a fixed head for writing a clock track onto one disk surface. Since direct access is required to the interior of the head-disk assembly of each disk drive unit, the servo writer is typically located within a so-called "clean room" in which the air is purged of impurities that might otherwise interfere with head-disk operations including the servo writing process. In one example, for a disk drive having two disks (four data storage surfaces) and requiring three servo-writer-controlled passes of the head over a single track during servo writing, total servo writing time might consume as much as 13.2 minutes. Thus, servo writing using servo writers in clean rooms requires either considerable capital investment in the manufacturing process or severe time penalties in the manufacturing process attributable to servo writer bottleneck. One very serious drawback relating to servo writers is that as track densities increase with evolving hard disk designs, servo writers become obsolete, and have to be replaced, or upgraded, at considerable capital expense.
This problem has not gone unnoticed in the art. One solution, proposed by workers at IBM, called for servo writing a master pattern at full resolution on one surface of a master disk during a pre-assembly operation. Then, a master disk with the master pattern was assembled with other blank disks into a disk drive unit. After the disk drive unit had been sealed against the ambient, the master servo pattern of the master disk was used as a reference by the disk unit in self-writing embedded sector servo patterns on each other data surface within the enclosed unit. Finally, the master pattern was erased; leaving the disk drive unit with properly located embedded servo sector patterns on every surface, including the surface which originally included the master pattern. This servo writing method is described in U.S. Pat. No. 5,012,363 to Mine et al, entitled: "Servo Pattern Writing Method for a Disk Storage Device", and is further described in a technical paper by Hiroyuki Ono, one of the named co-inventors of the '363 patent, in "Architecture and Performance of the ESPER-2 Hard-Disk Drive Servo Writer", IBM J. Res. Develop. Vol. 37, No. 1, January 1993, pp. 3-11. One of the noted drawbacks of the IBM approach is that the master unit was servo written on a different spindle than the disk drive spindle, and certain repeatable run out information had to be removed during the self-servo write operation. Another obvious drawback of the IBM approach is that some number of expensive servo writers would still be required to write the master patterns on some of the disks.
At the other end of proposed solutions is a complete disk drive self-servowrite operation. One such approach is described in commonly assigned U.S. Pat. No. 5,668,679 to Swearingen et al., entitled: "System for Self-Servowriting a Disk Drive", the disclosure thereof being incorporated herein by reference. The method of the '679 patent essentially comprises the steps of writing a clock track at an outside diameter (OD) recording region of a first disk surface of a disk drive having multiple storage surfaces, tuning an open-loop seek from OD to an inside diameter (ID) recording region to develop a repeatable seek profile, and recording a plurality of high frequency spiral tracks from OD to ID, each spiral track including embedded (e.g. missing bit) timing information. Then, spiral track provided peak data, and missing bit data, are read back. A voltage-controlled oscillator is locked to the timing information to track disk angular position. As the head is then moved radially from OD to ID the detected spiral peaks shift in time relative to a starting (index) mark, although the timing information does not shift. Embedded servo sectors can then be precisely written across the data storage surface by multiplexing between reading spirals and writing servo sectors (wedges). After the integrity of the wedges has been verified, the spirals are erased (over written with user data). While this system has been made to work well, challenges remain in generating and recording an accurate clock pattern on the first disk surface, and also in the time required to produce the master position pattern, on the first disk surface.
With the known drawbacks of servo writers and with self-servo writing, magnetic printing offers the possibility of a considerable improvement in the servo writing process. Magnetic printing comprises a direct transfer of magnetic patterns to a disk via a magnetic pattern or die, or by way of local heating above the Curie temperature as by laser beam, etc. One well-known and generally undesirable manifestation of magnetic printing is the "print-through" phenomenon. This phenomenon has been noted and explored, particularly in the field of magnetic audio recording tapes, see, e.g. Bertram, et al., "The Print-Through Phenomenon", Journal of the Audio Engineering Society, Vol. 28, No. 10, October 1980, pp. 690-705. While print-through of information recorded on magnetic tape at audio rates has resulted in annoying playback images occurring before and after the main recording, print-through has demonstrated the existence of the magnetic printing phenomenon for directly transferring a pattern recorded on a first magnetic medium onto a second magnetic medium brought into intimate contact with the first. Heating the second magnetic medium to approach or exceed the Curie temperature coupled with a flux-directing externally applied field is also known to facilitate direct transfer of magnetic patterns to a copy. While magnetic printing would seem to be an immediate solution to the difficulties with servo writers and with self servo writing techniques, one chief drawback of direct magnetic printing is that this process is reported to be unable to transfer high density information, such as magnetic patterns carrying video information, see Mallinson, The Foundations of Magnetic Recording, 2d Ed., Academic Press, San Diego, Calif., @1993, p. 32.
It is known within the disk drive art to position a data transducer head on the basis of timing differences derived from radially staggered disk servo patterns. The commonly assigned U.S. Pat. No. 5,668,679 referenced above is one example of such usage. Other examples of timing-based servo patterns are provided by U.S. Pat. No. 4,157,577 to Porter, Jr., entitled: "Rotatable Storage Apparatus with Digitally Responsive Circuitry for Track Selection"; and, U.S. Pat. No. 4,488,187 to Alaimo, entitled: "Servo Control Apparatus".
While the foregoing is thought to represent the state of the art, there has heretofore been no satisfactory application of magnetic printing as a replacement technology to servowriters or as an improvement to self-servowriting.